The wave nature of light

Passing light through two closely spaced narrow slits produces something particles alone couldn't explain: an alternating pattern of bright and dark fringes on a screen behind them, exactly the kind of interference pattern you'd expect from two overlapping sets of ripples.

The bright fringes appear where light waves from the two slits arrive in step and reinforce each other, and the dark fringes appear where they arrive out of step and cancel — the exact locations of these fringes depend on the wavelength of the light, which is itself strong evidence that whatever light is, it has wave-like properties.

This double-slit result, first demonstrated by Thomas Young in 1801, settled a long-running argument in light's favour of being a wave rather than a stream of particles — though the story doesn't end there, since the photoelectric effect would later show light also behaves like a particle in other circumstances, an apparent contradiction resolved by wave-particle duality.